In many industries, such as medicine, electronics, precision machinery, microbiology and so on, a high air cleanliness is required for the factory building or ward. Generally, there are two index systems for the cleanliness of a clean room. One is the number of particles per unit volume of air, and the other is the number of microorganisms per unit volume of air. Moreover, for the first index, a laser airborne particle counter is usually used for the measurement. For the second index, instruments such as an airborne microbe sampler is usually used to sample the microorganisms in the air. Then, a cultivation and a count are performed. Finally, the number of microorganisms per unit volume of air is obtained based on the result by backstepping calculation.
For the detection of microbiological indicators in the air, the sampling & cultivation method is costly in terms of time and labor, especially when the measurement is performed frequently at multiple locations. In addition, since the sampling & cultivation method is not a real-time technical means, an on-line monitoring measurement cannot be realized. Moreover, the result is usually obtained after 24 hours or more. The time delay often brings trouble to the process such as quality control etc. during the production. To solve this problem, many new technologies are raised, and the most widely approved technology is the laser excitation biological fluorescence detection. Microorganisms in the air (mainly bacteria) generally contain fluorescent groups such as riboflavin, NADH, tryptophan, tyrosine, etc. When the microorganisms are exposed to the laser having a special wavelength, the microorganisms will emit the fluorescence having a specific wavelength. Thus, the on-line monitoring for a single microorganism can be realized by detecting and analyzing the corresponding fluorescence signal. The representative advocate of this technology is Jim. Ho (U.S. Pat. No. 5,895,922, Fluorescent biological particle detection system), who achieved a synchronous detection of particle size and fluorescence parameters of a single microorganism in the air using a pulsed laser and subsequently a continuous output semiconductor laser as light sources. However, the particle size data is calculated based on the aerodynamic particle size principle, so that a sheath flow sample injection method is necessary to constrain the air flow to be measured. In addition, the laser must be precisely adjusted to form two parallel beams in the travelling direction of the particles to be measured. Thus, the measurement flow is greatly limited. Also, the system complexity, volume and weight are increased. Jianping Jiang et al. (US patent NO. 20070013910A1, Pathogen and particle detector system and method) use the continuous output semiconductor laser as a light source to achieve a synchronous detection of a scattering particle size of single particle and fluorescence parameters. A forward direction scattered light signal of particles is used to achieve the measurement of particle sizes. An ellipsoidal reflecting mirror is used to receive the fluorescence signals to realized the determination of the biology of single particle. Since the forward direction scattered light of the particle is used as one of the measurement object, and the laser has a stronger energy in the forward direction, further processing such as attenuation, absorption etc. must be applied to the beams to eliminate the impact brought to the scattered light detection. In addition, in this patent, due to the requirement of optical measurement and optical signal collection, the bottom of the ellipsoid should be provided with an opening to enable the air flow to pass through the first focus of the ellipsoidal reflecting mirror. To realize this purpose, the reflecting mirror should have a certain diameter, which has an impact on the miniaturization of the instrument.